Cyclone V Device Handbook: Volume 2: Transceivers

ID 683586
Date 10/24/2018
Document Table of Contents CDR PLL in Manual Lock Mode

In manual lock mode, the LTR/LTD controller directs the transition between the LTR and LTD modes based on user-controlled settings in the pma_rx_set_locktodata and pma_rx_set_locktoref registers. Alternatively you can control it using the rx_set_locktodata and rx_set_locktoref ports available in the transceiver PHY IPs.

In LTR mode, the phase detector is not active. When the CDR PLL locks to the input reference clock, you can switch the CDR PLL to LTD mode to recover the clock and data from the incoming serial data.

In LTD mode, the PFD output is not valid and may cause the lock detect status indicator to toggle randomly. When there is no transition on the incoming serial data for an extended duration, you must switch the CDR PLL to LTR mode to wait for the read serial data.

Manual lock mode provides the flexibility to manually control the CDR PLL mode transitions bypassing the PPM detection as required by certain applications that include, but not limited to, the following:

  • Link with frequency differences between the upstream transmitter and the local receiver clocks exceeding the CDR PLL ppm threshold detection capability. For example, a system with asynchronous spread-spectrum clocking (SSC) downspread of –0.5% where the SSC modulation results in a PPM difference of up to 5000.
  • Link that requires a faster CDR PLL transition to LTD mode, avoiding the duration incurred by the PPM detection in automatic lock mode.

In manual lock mode, your design must include a mechanism—similar to a PPM detector—that ensures the CDR PLL output clock is kept close to the optimum recovered clock rate before recovering the clock and data. Otherwise, the CDR PLL might not achieve locking to data. If the CDR PLL output clock frequency is detected as not close to the optimum recovered clock rate in LTD mode, direct the CDR PLL to LTR mode.